The present invention relates to a method of cleaning a transfer drum accommodated in an image transferring device of an electrophotographic apparatus and located to face a photoconductive element, or image carrier.
An electrophotographic apparatus of the kind electrostatically forming a latent image on an image carrier which is implemented as a photoconductive element, developing the latent image to produce a toner image, transferring the toner image to a paper sheet, and fixing it has been proposed in various forms. An image transferring device installed in such an apparatus has a transfer drum which is located to face the photoconductive element for retaining a paper sheet thereon. The transfer drum is provided with a clamper section for clamping one end of a paper sheet, and a transfer film. The transfer drum, i.e., the transfer film provided thereon is cleaned by a transfer drum cleaning device. The cleaning device is made up of a fur brush for cleaning the transfer drum, a toner collecting roller rotatable in contact with the fur brush, and a blade for scraping a toner off the toner collecting roller. In such an electrophotographic apparatus, e.g., a color electronic copier, a paper sheet is clamped by the clamper section of the transfer drum and wound around the drum, i.e. transfer film. Then, the photoconductive drum and transfer drum are driven in a rotary motion at the same time. A cyan toner image, magenta toner image and yellow toner image are sequentially formed on the photoconductive drum and sequentially transferred, by a transfer charger, one upon another to the same position on the paper sheet. On completion of such image transfer, a separator in the form of a pawl separates the paper sheet from the transfer drum. Subsequently, a fixing device fixes the toner image on the paper sheet while driving the paper sheet out of the copier to a copy tray. A problem with this type of copier is that after the image transfer the toner on the photoconductive drum is apt to deposit on the areas of the transfer film of the transfer drum other than the area where the paper sheet is wound, resulting in the surface of the transfer film being smeared. The transfer drum, therefore, has to be sufficiently cleaned, especially when the paper size is changed from small one to large one. This need is satisfied by the transfer drum cleaning device.
Specifically, the fur brush of the cleaning device is held in contact with the transfer film on the transfer drum so as to cause the toner on the film to deposit on the brush. The toner collecting roller rotating in contact with the fur brush is applied with a bias voltage so that the toner is handed over to the roller, while the blade scrapes that toner off the roller. However, the prior art transfer drum cleaning device constructed and operated as described above has some problems left unsolved, as enumerated below.
(1) The fur brush having cleaned the transfer drum, or transfer film, has some toner remaining thereon despite the operation of the toner collecting roller.
(2) Since the fur brush contacts the entire surface of the transfer drum or transfer film while in operation, the toner deposited on the fur brush is apt to accumulate in recesses which are formed in the clamper section. This part of toner accumulated in the recesses would smear one or both sides of a leading edge portion of a paper sheet to be clamped next.
(3) When a paper jam occurs in the electrophotographic apparatus, the toner is left untransferred on the photoconductive drum. Then, after the paper jam has been reset, the photoconductive drum is cleaned by an exclusive cleaning device independent of the transfer drum cleaning device. At the same time, the transfer drum or transfer film is cleaned by the transfer drum cleaning device. While the transfer film is cleaned, the transfer charger continuously charges the transfer film. As a result, during the cleaning operation, a substantial amount of untransferred toner is handed over from the photoconductive drum to the transfer film. In the transfer drum cleaning device, the untransferred toner so deposited on the transfer film is difficult to remove and, therefore, often constitutes a cause of smear.
(4) While the fur brush adapted to clean the transfer drum or transfer film is made of an insulative material, it is rotated at a speed thirty times higher than the linear velocity of the transfer drum. A motor for driving such a fur brush, therefore, generates offensive noise, and the fur brush is not durable.
(5) A method of cleaning the photoconductive drum by using a conductive fur brush is known in the art. This kind of method is implemented by a precleaning charger which uniformly charges the photoconductive drum by a DC-biased DC or AC voltage, so that the remaining toner is removed by electrostatic induction. On the other hand, the surface portion of the transfer drum or transfer film that faces the photoconductive drum is charged by the transfer charger at the time of image transfer. A problem given rise to at this instant is that the transfer film has to be discharged before the image transfer, i.e., the potential of the transfer film has to be brought to substantially zero volt before image transfer begins. Should the transfer film be not discharged sufficiently, there would occur incomplete image transfer. Hence, every time a copying cycle is completed, the transfer drum or transfer film has to be cleaned and then discharged to substantially zero volt. Further, during image transfer the toner on the photoconductive drum deposits on the transfer film other than the area where a paper sheet is wound, as stated earlier. In this condition, when the transfer charger charges the film surface to intense positive polarity, for example, the toner deposited on the transfer film as mentioned above will be also charged to the same polarity as the film. Assuming that the potential of the transfer film is as high as positive 500 volts to several kilovolts, causing a chargeable brush into contact with the film surface will result in a discharge and, therefore, in a smear due to the discharge. The polarity of the toner deposited on the transfer film is apt to reverse, further aggravating the difficulty of cleaning. The DC-biased DC or AC voltage applied to the transfer charger would accelerate such a discharge and thereby make the smear problem more serious.